NASA Glenn aerospace engineer Bryan Palaszewski uses cryogenic liquid nitrogen to cool the air inside a balloon at NASA's Plum Brook Station. As the air is cooled, it changes from a gas into a liquid. The balloon deflates, because molecules in liquids are more closely packed together than molecules in gases. Since most high energy propellants like hydrogen and oxygen are gases at room temperature, NASA uses the same process to cool rocket propellant for space shuttles in order to keep the rockets small and cost effective.